Temporal variation for canopy transpiration and its cooling properties in a Quercus acutissima forest of suburban Nanjing

Abstract

Global warming and regional water resources shortage have become the focus of the global. Climate warming affects global hydrological cycles, and tree transpiration also affects climate warming. This study was conducted to analyze canopy transpiration of Quercus acutissima and to determine its cooling effect on surrounding environments. Continuous observation of the sap flow density and environmental factors, such as solar radiation, air temperature, air relative humidity, soil temperature, soil water content, used a sap flow meter and a mini weather station in Nanjing Dongshanqiao Forest Farm. Maximum sap flux density, peak daily sap flux density, transpiration heat flux, solar energy, consumed-energy coefficient, and canopy transpiration cooling was conducted according these data. Six trees of different diameter grade were observed. Results indicated that 1） the maximum sap flux density appeared at 13:00; the greatest peak daily sap flux density was 18.78 gcm-2h-1 in August, and the lowest was 13.49 gcm-2h-1 in June. 2） Canopy transpiration summed from May to October was 237.52 mm with the highest month being July. 3） July also had the greatest transpiration heat flux （154.97 MJ m-2） and solar energy （489.65 MJm-2）. The coefficient of consumed-energy for canopy transpiration was highest in August （35.8%） and lowest in October （25.8%）. 4） Canopy transpiration cooling from Q. acutissima was highest in July （3.07 ℃）, and during the growing season, canopy transpiration reduced the average temperature （2.35 0.53）℃h-1 per 10 m3 air. According to the transpiration and its cooling effect, we could choose suitable afforestation design in order to alleviate global warming and heat island effect ［Ch, 6 fig. 1 tab. 28 ref.］